This invention relates to a radiation detector most adapted to be used with a computerized tomography.
A computerized tomographic apparatus for displaying the sectional plane of a human body by applying radiation such as X-rays is known as the apparatus for examining the human body.
This computerized tomographic apparatus fundamentally comprises, as shown in FIG. 1, and X-ray source 4 for emitting a fan-shaped X-ray beam in the form of a fan to surround a foreground subject 2 and a detector 6 fomred of a large number of X-ray detecting cells. The X-ray source 4 and X-ray detector 6 are rotated around the foreground subject 2 in the same direction at the same circumferential speed, thereby irradiating X-rays in various directions. X-ray photographed data on the foreground subject 2 which have been detected by the detector 6 are converted into electric signals. The converted electric signals are analyzed by an electronic computer, thereby figuring out X-ray absorption rates. An image of sectional plane of a foreground subject is constructed again in accordance with tone levels corresponding to said X-ray absorption rates.
The detector holds a large number of radiation-detecting cells defined between a pair of electrode plates. Each radiation-detecting cell constitutes an ionization chamber. X-ray energy entering the radiation-detecting cell through an X-ray permeable window is detected in the form of ionization current. Ionization current running through each X-ray path are integrated for a prescribed length of time. The integrated ionization currents are discharged in a discharge circuit having a prescribed time constant. The time of said discharge is taken as X-ray photographed data for each X-ray path. When collection of data at a position denoting a given circumferential angle is brought to an end, then collection of data is commenced at a position of the succeeding circumferential angle.
To elevate the radiation-detecting property of such a computerized tomographic apparatus as described above, it is necessary to enable the detector to absorb introduced radiation at a high rate. To this end, therefore, the detector is filled with a very stable heavy element having a large X-ray absorption coefficient, for example xenon or krypton with high pressure. Further to assure the stable detection of the detector, it is necessary that where the detector receives a fixed dosage of radiation, the respective cells constituting said detector be so arranged as to always send forth the same output. However, it has been impossible to fix the property of several hundreds of cells constituting the detector. Particularly, irregularities in the width of the interval between the electrode plates and the inclination of said electrode plates seriously affect the property of the detector cells. To date, therefore it has been impossible to realize a stable detection property by any other means than by manufacturing a detector with as great care as possible to equalize the property of the respective cells. Even if a single cell included in several hundred cells has a different X-ray detection property from that of the other cells, then a reconstructed image is contaminated by an artifact. At present, therefore, it is most earnestly demanded to develop a detector having an extremely stable property.